This invention relates to the manufacture of high performance solid oxide fuel cells (SOFCs), and, particularly to fabrication methods for electrode-supported fuel cells.
A fuel cell is a device which electrochemically converts chemical energy into electricity. Currently, there are two basic cell constructions for solid oxide fuel cells: electrolyte-supported cells and electrode-supported cells. In certain planar SOFC's with electrolyte-supported cells, the electrolyte is the mechanical support structure of the cell, with a thickness typically between 150 and 250 μm. In electrode-supported cells, the support electrode provides an electrical flow path, mass transport, and mechanical strength. Generally, the features and/or characteristics of the SOFC support electrode include an electrically conductive component, an oxide ion conducting component, and porosity. In addition, the support electrode has considerable thickness in order to provide the required cell flatness and mechanical strength. In SOFCs of this type, the electrolyte is comprised of a thin film, 50 μm or thinner, and is formed on the support electrode. Tubular, segmented-cells-in-electrical-series, and certain planar SOFC designs employ this type of cell. The use of a thin electrolyte in electrode-supported cells reduces the ohmic losses in the cells. Challenges remain, however, in improving electrochemical and mechanical performance of the support electrode.
U.S. Pat. No. 6,228,521 discloses an improved method for producing a high performance SOFC having a graded or multi-layered, relatively thick anode. More specifically, a Ni and YSZ anode is fabricated such that a major layer initially has about 80% by volume of NiO and a minor layer initially has about 60% volume NiO. The invention permits the use of thicker and thus stronger anodes without sacrificing electrochemical performance. Although the high amount of NiO in the major layer is preferred for the benefit of sufficient electronic conduction and porosity, fabrication of this configuration is problematic for large and flat cells because of a coefficient of thermal expansion (CTE) mismatch between NiO and YSZ. Further, the large volume of Ni in the anode after reduction could potentially cause anode creeping/sintering under high operating temperatures.
U.S. Pat. No. 5,270,536 discloses a method of fabricating a solid oxide fuel cell electrode, and in particular the anode. The method comprises forming a micro-composite element comprising a layered pattern of electrical conductive tape; creating a plurality of micro-composite subelements from the micro-composite element, each microcomposite subelement having the layered pattern; and juxtaposing at least two of the microcomposite subelements such that the layered patterns of adjacent microcomposite subelements are in different orientations relative to one another. The network formed with the subelements in the anode tape minimizes the randomness of anode components to thereby maximize electrical connectivity. According to the patent disclosure, the conductive network can also strengthen the overall structure of the anode while preventing dimensional changes that might otherwise occur during the anode reduction. Greater strength is achieved by virtue of the electrolyte network within the anode structure. One of the challenges of this method is to effectively control and keep the subelements in the desired order, as they are vulnerable to distortion forces during the manufacturing process.
World patent application WO02/058169 discloses an SOFC capable of being operated with sulfur-containing hydrocarbon fuel, as well as methods of fabrication of such fuel cells. One of the key features is the replacement of Ni in the conventional anodes with Cu- and ceria-containing materials. In order to generate enough porosity in the support anode structure so that it can be efficiently impregnated with catalyst materials, the anode structure made of NiO/YSZ is first reduced to Ni/YSZ and then refluxed with HNO3 to leach out the Ni. The resultant YSZ skeleton is then impregnated with catalysts such as Cu and ceria for sulfur-containing hydrocarbon fuel to avoid carbon deposition. However, the impregnation is often insufficient to supply adequate electrical flow paths.
In electrode-supported fuel cells, the support electrode needs a minimum thickness to provide sufficient mechanical strength. Generally, the thicker the support electrode, the stronger the cell. However, the cells with a supported electrode usually show bowing/non-flatness due to CTE mismatch and sintering shrinkage mismatch of electrode and electrolyte components. In electrode-supported SOFCs, thicker support electrodes improve the cell flatness. On the other hand, the thicker support electrodes could restrict mass transport through the electrode, e.g., limiting the oxygen transport in the support cathode or fuel/product transport in the support anode. The limitation on mass transport has a significant impact on fuel cell efficiency, as the concentration polarization will greatly reduce the fuel cell performance at high fuel and/or air utilization. Potential approaches to reduce the concentration polarization in the support electrode include:                Increasing the porosity in the support electrode;        Reducing the thickness of the support electrode; and        Designing and optimizing the pore/component structure of the support electrode.        